Boundary effect of mechanical waves of a blunt trauma can be exploited for reducing amplitude of the mechanical waves delivered to a brain tissue, using a multi-layered protective shell to increase number of boundaries inside the protective shell of a protective headgear as practically many as possible to a point there would not be a serious tissue injury to the brain tissue. Separately in a model of a two-layer medium panel with a first layer adjoining a second layer without a gap, it is known that there is no phase change at a boundary between the first layer and the second layer having a lower hardness than that of the first layer in reflected mechanical waves from incident mechanical waves traveling from the first layer to the second layer. Combination of both the incident and reflected mechanical waves in phase with each other temporarily increases an amplitude of the incident mechanical waves which increases an amplitude of transmitted mechanical waves in the second layer from the incident mechanical waves. If a series of the incident mechanical waves impacts the first layer, an amplitude of the reflected mechanical waves off the boundary merges with an amplitude of successive mechanical waves following a first wave of the mechanical waves coming toward the first layer. The amplitude of the successive mechanical waves following the first wave of the mechanical waves temporarily increases upon the addition of the amplitude of the reflected mechanical waves in phase with the successive mechanical waves, which increases a magnitude of an impact of the successive mechanical waves following the first wave of the mechanical waves to the second layer. If the first layer is made of a material that has a lower hardness than that of the second layer, the reflected mechanical waves off the boundary between the first and the second layers from the first wave reverse the phase and merge with the successive mechanical waves coming toward the first layer in a way the amplitude of the successive mechanical waves decreases. It results in a reduction of the magnitude of the impact of the successive mechanical waves to the second layer.
Intensity of an incident mechanical force of collision delivered to the brain tissue depends on a mass (weight) of a source moving to the brain tissue of a victim, generating the incident mechanical force from a velocity of an impact from the source multiplied by a mass (weight) of the victim and a stopping distance of the impact by the victim colliding with the source: KE=½×mv2 where KE is kinetic energy before an impact, m is mass in kg and v is velocity in meter/second. Separately, time (duration) of the impact of the incident mechanical force on the brain tissue is another factor determining an extent of damage to the brain tissue. This entity, “Impulse of force” follows Newton's second law, i.e., Faverage (Average impact force)=mass×change in velocity during collision/change in time of collision, and is found to be equal to a change in momentum of the victim provided that the mass is constant. In a scenario of an impact by a moving source to the brain tissue where the change in momentum (Impulse of force) is fixed, and the impact stops the moving source, extending the time (duration) of the collision decreases a time average of the average impact force. Similarly, based on the work-energy principle, i.e., Faverage (Average impact force)×d (distance)=−½×mass×velocity2, extending distance moved during the collision reduces the average impact force. Since the stopping distance of the impact by the victim is a relatively fixed value and the velocity of the impact from the source could be a relatively fixed value depending on a type of collision, we need to substantially increase size of a protective headgear to achieve a meaningful reduction in the average impact force to the brain tissue of the victim, if we use the distance as a factor to reduce the average impact force. However, the time of the collision can be practically extensible with a time-delay device for the impact, without a need to substantially enlarge the size of the protective headgear. One such device is a pressurizable and ventable sac surrounding the multi-layered shell of the protective headgear (US 20170280813 A1), which releases a pressurized gas from the sac upon the collision over a certain period of time. An analogous mechanistic example can be found in a car tire that is fully inflated with a gas under pressure and is releasing the gas through a nail puncture hole each time the tire runs over bumps of rough patches of a road. Releasing the pressurized gas upon each collision with the bumps, the tire continues to absorb an impact from the collision until the pressurized gas gets depleted from the tire. Not only is the time of the collision extended by the pressurized gas inside the tire, but also a part of the average impact force is released along with a portion of the pressurized gas that is vented through the nail hole upon the collision.
In a system comprising a plurality of concentric layers for the multi-layered shell of the protective headgear, efficiency in reduction of the average impact force can be enhanced further by sequential release of the average impact force by an individual concentric layer of the multi-layered shell. A basic motif of the multi-layered shell for the sequential release of the average impact force comprises an outer layer, a mid layer and an inner layer. The mid layer is configured to be undeformable, and to serve as a separating barrier of a centripetal incident mechanical force of a colliding source from a centrifugal incident mechanical force from the victim's head. The mid layer comprises an outer ply of a hard thermoplastic polymer, a mid ply of a compressible polymer foam and an inner ply of a hard thermoplastic polymer. The outer layer comprises a pressurizable and ventable sac in a hemispherical bowl configuration having a compressible and deformable polymer filling up inside the pressurizable and ventable sac. The compressible and deformable polymer includes a structured configuration such as concentrically stacked-up polymer tubes, and an unstructured configuration such as an open-cell polymer foam. The outer layer having a pressurized gas inside is configured to vent the pressurized gas upon a collision through a plurality of valves that have a range of threshold pressure for venting. The inner layer comprises a plurality of compressible and deformable blocks made of polymer foams, with each block enclosed tightly by an outer membrane. The outer membrane is configured with a plurality of holes through which air passively moves in and out of the block upon decompression and compression of the block, respectively. The inner layer is configured to encircle the victim's head and to be centrifugally compressed by the centrifugal incident mechanical force from the victim's head upon the collision. Upon the collision with the colliding source having the centripetal incident mechanical force, the outer layer is configured to complete venting of the pressurized gas before the inner layer is fully compressed by the centrifugal incident mechanical force from the victim's head. In this sequence, a portion of the average impact force from the centripetal incident mechanical force is released from the outer layer over an extended time while the inner layer continues to release a portion of the average impact force from the centrifugal incident mechanical force beyond the extended time for the outer layer. This two-step staggered sequence of extension of time of the collision and differential release of the average impact force amplify reduction of the average impact force and help reduce summation of bidirectional incident mechanical forces inside the brain tissue, respectively.